1) Field of the Invention
The present invention relates to an image communication apparatus that reads a color image and encodes data associated with the read color image before transmission to a destination.
2) Description of the Related Art
Recently, a facsimile machine having functions for reading and printing color images or a facsimile machine capable of transmitting and receiving color image data (color facsimile machine) has been widespread.
When the color facsimile machine is employed to transmit a color image in the art, read digital RGB signals (multi-valued data of R, G, B) are converted into color image data L*, a*, b* in LAB space and compressed before transmission to the destination.
When the color facsimile machine is employed to transmit an image, a color image is sent to the destination when it is a color facsimile machine. A monochromic image (binary image) is sent to the destination facsimile machine when it is a facsimile machine that has a function only for transmitting and receiving monochromic images (monochromic facsimile machine).
The transmission may be performed by the memory transmission. In the memory transmission, a draft image is read, a connection with the destination facsimile machine is established, information on different functions provided in that facsimile machine is acquired, and the draft image is transmitted.
FIG. 11 is a functional block diagram that shows a conventional image communication device 9. The image communication device 9 comprises a scanner 901, an A/D converter 902, a video compensator 903, an image quality processor 904, a transmission image generator 905, a coder 906, a storage section 907, a communicator 908 and a controller 909. This image communication device 9 is a color facsimile machine that has a network scanner function for compatibly connecting to a telephone line as well as to a LAN.
An operation of the image communication device 9 to perform color space conversion will be explained below with reference to the flowchart shown in FIG. 12. The scanner 901 reads a color draft image and generates analog RGB signals (step S901). The A/D converter 902 converts the analog RGB signals into digital RGB signals (multi-valued digital data of R, G, B) (step S902). The video compensator 903 performs shading compensation (step S903). The image quality processor 904 performs edge emphasis by a filtering operation (step S904).
Through the use of the color space conversion function of the transmission image generator 905, the digital RGB signals or color image data R, G, B in the digital RGB space are converted into color image data L*, a*, b* in LAB space (ITU-T standard data format for image communication devices) (the first color space conversion, S905).
The coder 906 converts (encodes) the color image data L*, a*, b* into JPEG image data (step S906). The storage 907 temporarily stores the JPEG image data (step S907).
The transmission image generator 905 has a transmission button; and when that transmission button is pressed down, the communicator 908 establishes a connection to the destination image communication device. The controller 909 can acquire information on a transmission resolution as well as information on the destination image communication device. The information on the destination image communication device is the information that indicates whether the destination is a device having a function for processing color image data in LAB space (generally an image communication device of a telephone line connection type) or a device (image communication device) of a network connection type having a function for processing color image data in sRGB space such as an Internet facsimile machine. This information acquirement is performed when the communicator 908 establishes a connection to the destination image communication device.
The controller 909 confirms whether a resolution conversion or a color space conversion is required according to the destination image communication device (step S908). When these conversions according to the destination are not required, JPEG image data stored in the storage 907 is transmitted to the destination image communication device by the communicator 908 (step S915).
When the resolution conversion or the color space conversion is required, the coder 906 decodes the coded data (step S909). First, when the color space conversion is required (step S910), through the use of the color space conversion function of the transmission image generator 905, LAB color space is converted into sRGB color space according to the destination image communication device (the second color space conversion, S911). When the resolution conversion is required (step S912), the transmission image generator 906 performs conversion of the resolution according to the destination (step S913). Then, the coder 906 re-encodes the color image data in sRGB color space to JPEG mode (step S914). The communicator 908 transmits the color image data thus re-encoded to the destination image communication device (step S915).
As for the memory transmission, it is not obvious before image reading when the destination is a color facsimile machine or a monochromatic facsimile machine. Therefore, the source color facsimile machine reads a draft image in a color mode and accumulates it in a memory unit. Then, after completion of connection to the destination facsimile machine, the color image data accumulated in the memory unit is sent when the facsimile machine is a color facsimile machine. In contrary, when it is a monochromatic facsimile machine, the color image data is converted into binary image data before it is transmitted.
When the destination image communication device is an image communication device of a network connection type such as an Internet facsimile (a device having a function for processing color image data in sRGB space), the image communication device 9 shown in FIG. 11 performs color space conversions twice in total, (1) a conversion from RGB color space into LAB color space by the scanner 901 at the time of reading a draft, and (2) a conversion from LAB into sRGB by the communicator 908 at the time of transmitting color image data.
As a result, an operational error due to the color space conversions (color space conversion operational error) increases and causes image quality deterioration such as variations in the color tone. In particular, when hardware is employed to configure the color space conversion function of the transmission image generator 905, it is generally required to simplify the hardware configuration suitable for an approximate operation mode, which increases the operational error.
The current international standard for color facsimile machines defines JPEG as a compression mode for transmission images. Therefore, it is convenient to accumulate draft images in JPEG format in a memory within a color facsimile machine as image data. Though, as the normal JPEG is of an irreversible compression type, it is impossible to restore to the draft image after encoding. The higher the compressibility of the JPEG image is increased, the more the image data can be accumulated in the memory within the color facsimile machine. In contrast, the image quality is remarkably deteriorated in the restored image.
In the image communication device 9 shown in FIG. 11, when the communicator 908 is employed for memory transmission and the destination is a monochromic facsimile machine, the transmission image generator 905 is required to convert the color image accumulated in the storage 907 into a binary image. Even when the color image is encoded into JPEG data at a level of permissible image quality deterioration on color image transmission, when it is converted into a binary image, an influence from the deteriorated image quality due to coding may appear in the restored binary image as a noise. Specifically, when the draft image is converted into JPEG image, characteristic noises named block noise and mosquito noise can appear at a higher compressibility on the edge of the restored image at obvious locations (specifically, on contours of characters, for example). Such the noises are remarkable particularly in the binarized image and dot-like noises appear around characters in the restored binary image (the image printed by the destination monochromatic facsimile machine).
A color draft image may be converted into a binary image at a certain resolution. It is also converted into a multi-valued image (color image) at the same resolution. In a comparison of both cases, an amount of information on visual identification when converted into the binary image is extremely less than that when converted into the color image. For example, even a sufficient resolution for facsimile transmission of a color image may lower the amount of information and reduce the visual identification when the brightness data is binarized at the same resolution.